Influence Modeling of muddy phosphate mining waste discharge on hydrodynamics in the Beninese-Togolese coastal waters and transport of waste phases

The Kpémé plant (Togo) discharges phosphate-rich muddy waste laden with heavy metals (Cr, Cd, Hg, etc.) into the coastal ocean. These discharges, composed of heavy apatite-rich phases and light clay-rich phases, disrupt hydrodynamics, degrade water quality, contaminate ecosystems, and threaten human health via bioaccumulation. This study employs the Delft3D suite (FLOW, WAVE) to model the transport of waste phases (density, particle size) and their impacts along the Beninese-Togolese coastline. Simulations integrate bathymetric data (GEBCO), meteorological inputs (ERA5), in situ measurements, and span two key seasons (May and August 2024) to predict dispersion, sedimentation, and ecological risks. The assessed extended seasons revealed distinct hydrological contrasts along the Beninese-Togolese coast. Current velocities and wind-driven wave heights were higher during the cold season (depth-averaged velocities up to 0.272 m/s; significant wave heights up to 0.70 m), promoting enhanced dispersion of light clay-rich phases toward deeper waters. Sediment transport increased during the cold season, with average depth flow rates rising by + 76.77 m³/s compared to the warm season. Heavy apatite-rich phases accumulated nearshore (0.97–305.90 m³/s), particularly along Togolese coastal zones, while light clay-rich phases migrated toward oceanic depths (0.61–39.28 m³/s), forming extensive turbid plumes. Sediment transport elevated turbidity, degraded benthic habitats, and dispersed heavy metals. Seasonal dispersion of waste phases was amplified during the cold season. Heavy apatite-rich phases predominantly impacted coastal zones, whereas light clay-rich phases contaminated deep-sea ecosystems. This modeling underscores the urgent need for mitigation strategies (e.g., waste pretreatment, monitoring) to safeguard biodiversity and public health.